Rock fern poisoning is capable of causing significant disease in both sheep and cattle. These ferns have been associated with two main syndromes in cattle. Osteomyelotoxic fern glycoside poisoning is an acute form of poisoning which causes extensive haemorrhage. A more chronic syndrome also exists where carcinogenic effects of the toxin cause benign or malignant tumours of the urinary bladder, also known as bovine enzootic haematuria (McKenzie 2012). In Australia, sheep grazing rock fern develop either a haemorrhagic disease or more commonly polioencephalomalacia, a condition associated with ataxia, laboured breathing and collapse (Chick et al. 1981).
The rock ferns Cheilanthes distans (woolly cloak fern) and C. sieberi (mulga fern) are small upright plants with short, heavily indented, roughly triangular leaf blades and dark or red brown stems. These ferns are native to Australia and grow to an average height of 20cm (Beckett 1984). C. distans can grow up to 30cm tall with leaf blades approximately 1.5 cm long and coarse scales present on the under surface of the leaves. C. sieberi can grow up to 35cm tall and its leaf blades are up to 2cm in length without hairs. C. distans occurs mainly in coastal districts in habitats which contain exposed rock. C. sieberi grows in hilly and rocky areas and in woodlands across arid, semi-arid and coastal districts, particularly beneath Arcadia aneura (mulga) trees (McKenzie 2012). Rock ferns are adapted to dry conditions as they are capable of extreme desiccation during periods of drought, whilst still remaining viable (Clark &Dimmock 1971). They may be the first green plants to reappear after a period of drought (McKenzie 2012). Thus the fern can become an attractive green feed to hungry stock before other pasture species germinate and grow (Beckett 1984).
Ptaquiloside is the most prominent toxin present in Cheilanthes spp. and belongs to a family of compounds known as the norsesquiterpene glycosides. Ptaquiloside has been measured in C. sieberi at concentrations of 150 to 448mg/kg DM in Australia and New Zealand. Ptesculentoside and caudatoside are other compounds that have isolated from rock ferns. These norsesquiterpene glycosides are found in the fronds and become most concentrated in young unfurling fronds (McKenzie 2012).
Osteomyelotoxic fern glycoside poisoning has been reported since the late nineteenth century (Gil da Costa et al. 2012). Cattle are most often affected by this syndrome; it is very rare in sheep. Sheep exhibit similar haematology and pathology to cattle but they are more resistant and disease takes longer to develop (Clark &Dimmock 1971). Animals need to eat a large amount of ferns and calves are more likely to be affected than adults (McKenzie 2012). The toxins are cumulative in their effects and ferns need to be grazed for several weeks before signs of poisoning can occur (Beckett et al. 1984). The norsesquiterpene glycosides target rapidly dividing cells such as the bone marrow and intestinal mucosa (Beckett 1984). This destroys the ability to produce platelets and neutrophils (McKenzie 2012), which manifests haematologically as leukopaenia and thrombocytopaenia (Beckett 1984). Interestingly, cattle with the disease are more likely to exhibit severe neutropaenia, whilst sheep will present with a marked lymphopaenia (Gil da Costa et al. 2012). Clinically, animals have pale mucous membranes and petechial or larger haemorrhages of the mucous membranes and sclera (Beckett, 1984). There may also be melena, haematuria, epistaxis and a muco-haemorrhagic nasal discharge. Secondary bacterial infections can occur late in the course of the disease (McKenzie 2012). Bacteraemia and subsequently embolic infarcts of bacteria may occur as a result of severe neutropaenia (Beckett 1984). Post-mortem often reveals sub-serosal haemorrhages of the abdominal organs and extravasations of blood in the abdominal cavity (Beckett 1984). Microscopy of the bone marrow demonstrates lack of megakaryocytes (McKenzie 2012). Experimentally, 8g of C. sieberi fronds/kg BW/day for at least 10 days was needed to poison young cattle (McKenzie 2012). Ptaquiloside has been isolated as the aetiological agent causing disease in cattle (Gil da Costa et al. 2012). No effective specific treatment is available. Notifiable differential diagnoses to be aware of include anthrax and haemorrhagic septicaemia (Pasteurella multocida) (Robinson & Moloney 2008; Shivachandra et al. 2011). Anthrax is an especially important differential to exclude given it has zoonotic potential and is endemic to the central west of NSW (Hendry 2012).
Bovine enzootic haematuria is a more chronic disease than osteomyelotoxic fern glycoside poisoning (Beckett 1984). It has been associated with cattle feeding on toxic ferns over 2 or 3 years (Gil da Costa et al. 2012). The condition is usually fatal. Clinically, an intermittent haematuria is seen which progresses to persistent haematuria and chronic anaemia. The disease is characterised by haemangiomatous lesions of the urinary bladder wall. These neoplasms may vary from sessile polyps, fibromas and papillomas to haemangiomas, sarcomas and carcinomas, with papillomas and carcinomas of the transitional epithelium being most common (Beckett 1984). Ptaquiloside has been shown to induce bladder tumours in laboratory animals, and is believed to be the causal agent of bovine enzootic haematuria (Gil da Costa et al. 2012). Prevention of both osteomyelotoxic fern glycoside poisoning and enzootic haematuria can be achieved by controlling growth of rock ferns through repeated slashing or using selective herbicides, in combination with pasture improvement (McKenzie 2012).
Polioencephalomalacia is a disease mainly of ruminants that is thought to be caused by anoxic damage to cerebrocortical neurons as a result of thiamine deficiency or alterations to thiamine metabolism (Chick et al. 1981). Thiamine (vitamin B1) is necessary for metabolic reactions in the Krebs cycle and for maintenance of myelin membranes (Chick et al. 1981). Ruminants generally have their thiamine needs met by production from rumen microflora (Finnie et al. 2011). Consumption of rock fern by sheep results in the ingestion of thiaminases, which are enzymes that cleave the thiamine molecule, and render it biologically inactive (McKenzie 2012). Rock fern is classified as a type 1 thiaminase as it acts to displace the pyrimidine methylene group with a nitrogenous base or SH compound, to eliminate the thiazole ring (Finnie et al. 2011). Once the thiamine molecule becomes cleaved, the body is incapable of restoring it (Chick et al. 1981). Polioencephalomalacia in ruminants manifests as disorientation, wandering, blindness, opisthotonus, teething grinding, muscle tremors, head pressing or retraction of the head (McKenzie 2012). The brain of infected animals becomes inflamed and oedematous. Animals that are handled are more likely to collapse and suffer convulsions. Death usually occurs two to four days after the onset of clinical signs (McKenzie 2012).
Economically, the impact of fern poisoning in cattle can be significant. The osteomyelotoxic form and enzootic haematuria forms of the disease are both untreatable and result in death of the animals (Beckett 1984). Rock ferns are also potentially toxic or carcinogenic to humans, and may be acquired indirectly through the ingestion meat or milk from animals grazing affected pastures (Beckett 1984).
LHPA Central West Case
Signalment: Early weaned dairy calves placed in pasture with no mature animals.
Problem: All calves in the paddock had died (3 in total)
Pasture: On examination of the pasture a large quantity of rock fern was found. Close examination revealed that some plants had been stripped of leaves (see Figure 1). There had been recent rainfall in the area and subsequent growth of the rock ferns.
Post-mortem findings: Multifocal ecchymotic and larger sub-dermal haemorrhages are visible on reflexion of the skin from the thoracic cavity (Figure 2). Multiple internal organs have extensive subserosal haemorrhage (Figures 3 & 4). In the pleural cavity there are fibrin tags off the pleural surface and a marked accumulation of partially clotted blood (Figures 5 & 6). The surface of the gall bladder is markedly reddened and haemorrhagic (Figure 7). The bladder contained port wine coloured urine prior to evisceration and the lumen of the bladder is petechiated and haemorrhagic (Figure 8). Petechiae are also apparent on the luminal surface of the abomasum (Figure 9).
Morphological diagnosis: Marked acute extensive multifocal haemorrhages
Aetiological diagnosis: Osteomyelotoxic fern glycoside toxicity
The incidence of rock fern poisoning in the central west of NSW has increased over the last two years. This may be due to an increase in animals coming into the area who are naive, or because environmental conditions have favoured growth of the plants (E Walker, 2013 pers. comm., 8 April). Most cases seen around the Dubbo district are associated with hand reared dairy calves that were recently weaned and placed in a pasture with no mature animals (G McCann, 2013 pers. comm., 10 April). Other cases have been seen in both adult animals and calves that were recently introduced to the area (G McCann, 2013 pers. comm., 10 April; A Taylor, 2013 pers. comm., 10 April). It is possible that these were affected because they were naive to the ferns, and had not learned to graze as selectively as animals raised locally. In addition, there is a tendency for it to occur on hobby properties. This could be because the rock fern is not controlled, the manager is unaware of its toxic effects, or because hobby farmers are more likely to bring naive animals into the area (A Taylor, 2013 pers. comm., 10 April).
In adult animals one of the first signs seen is ulceration of the muzzle (G McCann, 2013 pers. comm., 10 April). Adults also present with epistaxis and melena, and can often mistakenly be diagnosed as anthrax outbreaks, until further diagnostics rule the disease out (A Taylor, 2013 pers. comm., 10 April).
Mitigation of further cases associated with grazing rock fern can be a difficult task for property owners due to the fact that most are associated with hobby farms that have a poor quality soil type to begin with, have rocky outcrops scattered throughout the property and have limited infrastructure in the way of agricultural implements to address the issue.
In these cases, an alternative less susceptible species, like sheep or goats, is recommended (G McCann 2013, pers. Comm.., 2 July)
Producers should however be aware that a single slashing of bracken fern followed by grazing of the young fronds can increase the risk of poisoning (Ian Poe).
It is often not practical to control rock fern through slashing and herbicides because it grows in non-arable country. However, it rarely grows in cultivated country. It seems that cultivation alone will kill it. (Bruce Watt).